In recent years, crude oil and natural gas have been being actively mined in severe corrosion environments, which have been withdrawn from development, because of soaring crude oil prices, needs for the diversification of energy sources, and the like. Crude oil and natural gas are actively mined in oil and gas fields in humid environments containing hydrogen sulfide, carbon dioxide, chlorine ions, and/or the like or in frigid locations such as North Sea, Canada, and Alaska. Pipelines for transporting mined crude oil or natural gas are under active construction.
Pipelines tend to be operated at high pressure with large diameter for an increase in transport efficiency. In order to endure the high-pressure operation of such pipelines, transport pipes (line pipes) need to be thick steel pipes and UOE steel pipes made from thick steel plates have been used. However, high-strength welded steel tubes made from coiled hot-rolled steel sheets (hot-rolled steel strips) are high in productivity, are more inexpensive, and therefore have been recently used as transport pipes instead of such UOE steel pipes made from thick steel plates in accordance with strong needs to further reduce construction costs of pipelines. These high-strength welded steel tubes need to have excellent low-temperature toughness from the viewpoint of preventing the destruction of pipelines and further need to be excellent in so-called sour resistance including hydrogen-induced cracking resistance (HIC resistance) and stress corrosion cracking resistance in addition to these properties.
In order to cope with these needs, for example, Patent Literature 1 proposes a “method for manufacturing a steel sheet for steel tubes having excellent sour resistance”. A technique described in Patent Literature 1 is a method, for manufacturing a steel sheet for steel tubes having excellent sour resistance, including finishing the hot rolling of a continuously cast product containing 0.04% to 0.16% C, 0.6% to 1.8% Mn, one or more of 0.06% or less Nb, 0.07% or less V, 0.03% or less Ti, 0.50% or less Mo, and 0.50% or less Cr, and about 0.3% or less Si at a temperature of 750° C. or lower; immediately performing quenching by cooling to a temperature of 200° C. or lower at an average cooling rate of 20° C./s to 80° C./s; and then performing tempering at 450° C. to 750° C. According to the technique disclosed in Patent Literature 1, a high-strength hot-rolled steel sheet, dramatically enhanced in sour resistance, having a tensile strength of 540 MPa or more is obtained.
Furthermore, Patent Literature 2 proposes a “method for manufacturing a steel sheet for high-strength line pipes having excellent HIC resistance”. A technique described in Patent Literature 2 is a method, for manufacturing a steel sheet, including heating a semi-finished product which contains 0.03% to 0.08% C, 0.05% to 0.50% Si, 1.0% to 1.9% Mn, 0.005% to 0.05% Nb, 0.005% to 0.02% Ti, 0.01% to 0.07% Al, and 0.0005% to 0.0040% Ca and which satisfies a Ceq of 0.32% or more to 1,000° C. to 1,200° C.; performing two-stage cooling comprising accelerated cooling to a surface temperature of 500° C. or lower after hot rolling is finished; interrupting accelerated cooling once so that the surface is reheated to a surface temperature of 500° C. or higher; and then performing cooling to a temperature of 600° C. or lower at a cooling rate of 3° C./s to 50° C./s. According to the technique disclosed in Patent Literature 2, a steel sheet which exhibits excellent HIC resistance and SSC resistance even in sour environments and which has a high strength equivalent to X70 or higher is obtained.
Furthermore, Patent Literature 3 proposes a “method for manufacturing a hot-rolled steel sheet for sour-resistant high-strength electric resistance welded steel pipes”. A technique described in Patent Literature 3 is a method, for manufacturing a steel sheet for hot-rolled steel sheet for sour-resistant high-strength electric resistance welded steel pipes, including heating a semi-finished product which contains 0.02% to 0.06% C, 0.05% to 0.50% Si, 0.5% to 1.5% Mn, 0.01% to 0.10% Al, 0.01% to 0.10% Nb, 0.001% to 0.025% Ti, 0.001% to 0.005% Ca, 0.003% or less O, 0.005% or less N, and one or more selected from the group consisting of 0.01% to 0.10% V, 0.01% to 0.50% Cu, 0.01% to 0.50% Ni, and 0.01% to 0.50% Mo and which satisfies Px=[C]+[Si]/30+([Mn]+[Cu])/20+[Ni]/30+[Mo]/7+[V]/10≦0.17 and Py {[Ca]−(130×[Ca]+0.18)×[0]}/(1.25×[S])=1.2 to 3.6 to 1,200° C. to 1,300° C.; performing hot rolling at a finished rolling temperature not lower than (the Ar3 transformation point minus 50° C.); immediately starting cooling; performing coiling at a temperature of 700° C. or lower; and then performing slow cooling. According to the technique disclosed in Patent Literature 3, an electric resistance welded steel pipe with a high strength equivalent to Grade X60 or higher can be manufactured, a hot-rolled steel sheet with a thickness of 12.7 mm or more can be obtained, and an electric resistance welded steel pipe with high weld toughness can be manufactured using the hot-rolled steel sheet.